There is a problem of limiting transmission speed when transmitting a discrete information over communication lines.
The greatest possible transmission speed C in a communication line could be determined in accordance with the Shannon's formula:
                              C          =                                    Π              ·                              log                2                                      ⁢                                                            P                  C                                +                                  P                  Π                                                            P                Π                                                    ,                            (        1        )            where Π is a bandwidth of communication line, KHz; PC is a power of signal being transmitted, dB; PΠ is a power of interference in the communication line, dB.
For a telephone line having a bandwidth Π 3.1 KHz (300 Hz−3.4 KHz) and a ratio PC/PΠ=10000 (which corresponds to 40 dB), a greatest theoretically possible speed of transmission (and reception) of discrete information isC=3.1·103·log210000=40 Kbit/s.
Modem telephone modems of firms “Robotics” and “Motorola” ensure the information transmission and reception speed up to 33.6 Kbit/s at the ratio PC/PΠ=10000 (corresponding to 40 dB), which is indicative of implementing the transmission speed close to the potentially possible in accordance with the Shannon's formula.
Known is the method of information transmission, including steps of: at the transmitting side, forming an analog signal by means of converting a sequence of discrete digital information samples to the analog signal using a predetermined sampling function, and transmitting the formed analog signal to a communication line; and at the receiving side, receiving the transmitted analog signal from the communication line, and restoring from it the original sequence of discrete digital information samples (JP 10-098497 A, Int. Cl. H 04 L 27/10, 14.04.1998).
The information transmission and reception speed in such a method for transmitting an information depends on a method for converting a sequence of discrete digital information samples to an analog signal at the transmitting side, and a method for restoring from the analog signal the original discrete digital information samples at the receiving side, and this speed is limited by the utmost capacity of pulse-code modulation (PCM) equipment existing in the communication line.
When transmitting over a single communication line simultaneously two analog signals of the same power PC1=PC2 (i.e., in the case of decompression into two similar powers), the equation (1) could be rewritten as follows:
  C  =                    Π        ·                  log          2                    ⁢                                    P                          C              1                                +                      P            Π                                    P          Π                      +                  Π        ·                  log          2                    ⁢                                                  P                              C                2                                      +                          P              Π                                            P            Π                          .            
Then it follows that at a fixed power
  P  =                    P                  C          1                    2        +                            P                      C            2                          2            ⁢              (                              i            .            e            .                    ,                                    at              ⁢                                                          ⁢                              P                                  C                  1                                                      =                          P                              C                2                                                    )            the maximum information transmission and reception speed could be brought to C=3.1·103·log2 5000+3.1·103·log2 5000≈73 Kbit/s. Hence, in telephone lines, the decompression into two powers allows to increase the information transmission and reception speed and to realize the utmost capacity of existing pulse-code modulation (PCM) equipment, which is 64 Kbit/s.